The Younger Dryas, a period of cooling between roughly 13,000 and 11,500 years ago that interrupted the warming that ended the last deglaciation, is clearly visible in many Northern Hemisphere climate records. Whether such cooling extended into the Southern Hemisphere has been hotly debated.

Newnham and Lowe analyzed pollen from a peat bog in New Zealand and find evidence for a cooling event that began around 13,600 years ago and lasted approximately 1000 years. This interval of cooling occurred 600 years before the Younger Dryas, which suggests that this climate event may have registered first in the Southern Hemisphere and then propagated into the Northern Hemisphere. Such a delay in the appearance of the climate signal could implicate thermohaline circulation as the agent of transmission. Another possibility, which arises because of the uncertainties in comparing different types of records, is that these events were nearly synchronous, which would imply that an atmospheric (and thus virtually instantaneous) connection between the hemispheres transmitted the change in temperature. In any event, a delayed response to the Younger Dryas by the Southern Hemisphere appears to be ruled out. — HJS